like Ian says, it is caused by white space between php tags.
the error is reporting the problem at line 5:
output started at... /httpdocs/siicom_cart.php:5
most likely, there is a blank space at the end of the closing php tag on line 4:
"?> "
see this post for more details on troubleshooting and fixing the headers already sent error:
showthread.php?t=19285